The invention lies in the field of the packaging industry and relates to a method and an apparatus according to the generic parts of the corresponding independent claims, which method and apparatus serve for treating the inside surface of plastic bottles using a plasma enhanced process.
The term plastic bottle is used in the present description for a container made of a plastic material and having a body portion in the form of an upright cylinder with a circular or a non-circular cross section, a bottom portion on the one face of the body portion and a shoulder and neck portion with a relatively narrow opening on the other face of the body portion. Such containers are in particular bottles made e.g. of polyethyleneterephthalate (PET), polypropylene (PP) or high density polyethylene (HDPE), which bottles are e.g. produced by stretch blow moulding. Such bottles, for improving their gas barrier properties, are subjected to a coating treatment for coating their inner surface with a layer of silicon oxide in a plasma enhanced chemical vapour deposition process.
Other plasma enhanced processes used for treating the inside surface of plastic bottles are e.g. sterilizing processes or processes for activating or otherwise changing the surfaces.
Plasma enhanced processes for treating the inside surface of plastic bottles as well as devices for carrying out such processes have been described e.g. in the publications J. Weichart, B. Meyer, J. Mxc3xcller (Vakuum in der Praxis Nr. 1, pages 22-26, 1991), U.S. Pat. No. 5,521,351 (Wisconsin Alumni Research), JP-0853117 (Kirin Brewery), U.S. Pat. No. 5,378,510 (Polar Materials Inc.), DE-3632748 (Vereinigung zur Fxc3x6rderung des Instituts fxc3xcr Kunststoffverarbeitung in Industrie und Handwerk an der TH Aachen) and WO-95/22413 (Coca-Cola Company).
A plasma enhanced process for treating the inside of a bottle generally comprises the steps of reducing the pressure within the bottle, igniting and sustaining a plasma within the bottle by activating a suitable power source (DC, RF, HF, microwave) and flowing a suitable process gas or process gas mixture through the plasma. In most cases it is necessary to evacuate the room outside of the bottle also in order to prevent the bottle from collapsing. Advantageously the room outside the bottle is evacuated to a pressure which is approximately ten times lower than the pressure inside the bottle such preventing ignition of a plasma on the outside of the bottle.
Devices for carrying out such processes therefore comprise: a vacuum chamber, in which the bottle to be treated is positioned, means for evacuating the vacuum chamber and means for evacuating the inside of the bottle (usually two different reduced pressures), means for igniting and sustaining a plasma (e.g. generator for RF- or HF-frequency or microwave generator) and means for feeding the process gas into the bottle.
It is the object of the invention to provide a method and an apparatus for treating the inside surface of plastic bottles in a plasma enhanced process, e.g plasma enhanced chemical vapour deposition of a silicon oxide coating. Using the inventive method and apparatus is to make it possible to produce a high quality product in an economically feasible process and the inventive apparatus is to be easily upscaleable for a high capacity process.
This object is achieved by the method and apparatus as defined by the claims.
The inventive apparatus comprises a vacuum chamber and a substantially cylindrical microwave confinement to which microwaves generated by a microwave generator are coupled through one of the faces with the aid of suitable coupling means, whereby the function of the microwave confinement and of the vacuum chamber may be taken over partially or fully by the same apparatus part. It further comprises evacuation means and gas feed means. The microwave confinement is adapted to the shape of at least the body portion of the bottle as closely as possible. The microwave confinement, the coupling means and the microwave generator are designed and tuned such that the microwave confinement is excited in a TM resonance mode, i.e. by a transverse magnetic wave, in which the magnetic field does not have axial components. In addition a permanent magnetic field can be generated by providing stationary magnets which magnetic field is preferably such that within the bottle to be treated electron cyclotron resonance conditions are achieved.
The inventive apparatus is very compact and very simple. For upscaling, a plurality of one-bottle apparatuses is arranged in a row or in a matrix and all the one-bottle apparatuses are connected to a net of energy, vacuum and gas supply lines. Each of the one-bottle apparatuses may have its own vacuum chamber or the plurality of microwave confinements may be arranged in one common vacuum chamber.